LEACHING AND DISSOLVING 347
expose the soluble particles to the action of the solvent. In the leaching of sugar from beets however, just the opposite is sought. In the beet we have two types of solid particles, crystalloids and colloids. We wish to dissolve the crystalloids and prevent the colloids getting into our solution. Crystalloids will pass through permeable membranes whereas colloids will not, or only very, very slowly. Now the sugar beet is made up of small cells containing the sugar and other crystalloids in solution with the colloids and these cells have walls of a very fine membrane. Therefore if when slicing the beet these membranes are not damaged, when brought in contact with water the crystalloids will pass through the membranes but the colloidal matter will be held back thus keeping the sugar solution clear.
Another case of interest in the beet-sugar industry of the application of osmosis is in removing objectionable salts from, molasses. It is found in sugar refineries that certain salts are taken into solution which tend to prevent crystallization as they become more concentrated. A molasses will be obtained therefore containing a very high percentage of sucrose but on evaporation the sugar does not crystallize due to the effect of these contaminating salts. These salts however are highly ionized, while sugar is not. They will therefore diffuse through a permeable membrane much more readily than sugar. Molasses containing these salts is placed in contact with permeable membrane on the other side of which is water. The salts then tend to flow through the membrane to the water and thus their concentration in the molasses is reduced, while the loss of sugar through the membrane is small.
Continuous Agitation.—The art of conducting leaching operations involving agitation continuously, has been well known and practiced in various metallurgical operations for quite a number of years, but has only recently been introduced into the chemical field. To design a continuous agitation system, we must first know the amount of material to be treated in any unit of time and secondly, the time of contact necessary to effect complete dissolution. We would then provide sufficient volume in our agitators to retain the material for this period and would feed and withdraw material at an equal and uniform rate.
A continuous agitation system may be made up of one or a series of agitators, depending on the time of retention necessary. A great deal has been written on the theory of continuous agitation, and especially on the possibility of untreated material being discharged prior to complete extraction. Of course, it is apparent that the material is not retained intact for the computed period, but that this represents merely an average time of detention. In fact, a portion of the feed at any instant will be discharged almost immediately and on the other hand, from the purely theoretical standpoint, another portion will remain in the system an infinitely long time.
In actual practice continuous agitation is seldom attempted in a single tank, but a series of three, four or even more, smaller agitators used in order to minimize the "short circuiting" effect. The number of tanks necessary depends primarily on the period of detention necessary. Several articles have been published in which an attempt has been made to derive a mathematical relation between the time of contact, number of agitators necessary and percentage of extraction.
These articles lead one to believe that continuous agitation can only be effectively applied by employing a prohibitive number of agitators of large capacity and yet the actual results obtained during the past 10 years in hundreds of metallurgical and chemical plants do not corroborate this.